His flop c-bet is really big so it's possible we're behind already, that being said it's also possible we're ahead if he 3bets TT-JJ.

When the turn comes he suddenly bets a lot smaller, which puts us in a awkward position. It's possible he has the nuts with a diamond or AdKd, it's possible we're still behind and drawing dead against a bigger pair with a diamond, yet it's also possible we're ahead of TT-JJ with or without a diamond as well as AdKx AxKd hands (not sure he bets the flop so strong with these tho), and he likely won't fold them for those odds no matter what he was thinking when he made such a small bet.

With already a $3 overlay in the pot i think we have no play except jamming at that point, barring a better read on his preflop range and his bet size on the turn. If you can take out TT of his range and you don't think he bets AK with a lone diamond on that kind of flop, you can actually make an argument for folding (tho kaitz would kill me for even suggesting it :p).